Extrusion Cooking for Fortified Blended Foods (FBFs)

Extrusion is the process of forcing a material through a specifically designed opening. Material is continuously metered into an inlet hopper and then transported forward by the rotating screw. as the material approaches the die, there usually is an increase in pressure and temperature.

Mechanical disruption of the cell walls and starch of plant products occurs during extrusion cooking, facilitating digestion, and absorption. This mechanical breakdown of starches reduces the viscosity of gruels made from extruded cereals to enhance their caloric and nutrient density. The high-temperature heat treatment effectively pasteurizes the product. A good packaging is required to provide resistance to moisture and insects.

Dry extrusion is preferred for the production of Super Cereals. This type of extrusion does not require an external source of heat or steam. All the cooking is accomplished by friction capitalizing on the inherent moisture and/or oil for providing lubrication. The dry extruder can process material bearing 8-22% moisture without the need for drying the extrudate.

Wet extrusion is considered more appropriate for the manufacturing of Super Cereals plus.

The advantages of extrusion are summarised in the scheme next.

Other processing techniques which can apply for the thermal treatment of the raw materials are:

Roasting describes a process that dry cooks a cereal, legume, or oil seed. The resulting dry ground products can be mixed with sugar and oil and moistened to form porridge, a ball, etc. High processing temperatures produce a pleasant toasted flavour that improves palatability and inactivates enzymes and anti-nutritional factors but also denatures heat-labile vitamins.

Roasting can be accomplished by shaking the grains in a heated pan or by immersing and agitating them in hot salt or sand. Large-scale roasting equipment with improved thermal efficiency that requires only a moderate level of skill to operate is available. Roasting loosens the seed coats, making them easy to remove before the product is ground. Although roasting is one of the least expensive cooking processes, it is limited to whole grains and products with uniform piece sizes. Roasting dries grains and destroys much of the surface micro-flora, thus increasing the shelf life if the product is protected from moisture and insects. Nevertheless roasting is not a sufficient treatment to pre-gelatinise the starch, as a consequence the energy density or nutrient density of the meal will not be sufficient for children under 2.

Proper roasting is normally done at temperatures not exceeding 170°C. It is recommended to roast cereals at 140°C for about 10 minutes and pulses, legumes, oilseeds at 170°C for about 15 min. After roasting the material is allowed to cool down to ambient temperature to avoid further breakdown of nutrients.

Cereal paste drying is commonly done on the surface of hot drums or by spraying the paste into heated air. The resulting dry, precooked products form smooth gruels when they are reconstituted with water. Supplementation with non-fat dried milk or soy protein and vitamin-mineral mixtures improves the nutritional quality. Proper packaging is necessary to protect the product from insects and moisture, but it is costly because of low product density.

In this operation, food slurry is contacted with a hot, revolving drum to form a thin layer on the surface. After sufficient residence time to allow the evaporation of water the product is removed from the drum by a scraper device (called a doctor knife) located usually 1/2 to 3/4 of a revolution from the point of application. Typically wheat can be made at 40% whereas rice at 30% solids. The temperature of the heat exchanger surface is 100°C, ensuring that the starch component of the cereal is fully gelatinised and reducing the microbiological load in the finished product.

Factors affecting the rate of drying and final moisture content are:

Residence time on the drum

Surface temperature

Film thickness

The method of dehydration can only be applied to food slurries or liquid food systems and the product must be able to withstand high temperature-short time exposures without undergoing severe quality changes. It has been most successfully applied to drying of milk, soup mixes, and baby foods. This technique is the most expensive but also the most sophisticated to produce Fortified Blended Foods of high quality.

The validation of these techniques is necessary to the extent of ensuring that WFP requirements for product stability, product self-life, energy density and acceptability are met.